Makes a unique request id available to the
action_dispatch.request_id env variable (which is then
accessible through ActionDispatch::Request#request_id or the
alias ActionDispatch::Request#uuid) and sends the same id to
the client via the X-Request-Id header.

The unique request id is either based on the X-Request-Id header in the
request, which would typically be generated by a firewall, load balancer,
or the web server, or, if this header is not available, a random uuid. If
the header is accepted from the outside world, we sanitize it to a max of
255 chars and alphanumeric and dashes only.

The unique request id can be used to trace a request end-to-end and would
typically end up being part of log files from multiple pieces of the stack.